Project Summary Abstract Kaposi's Sarcoma-associated herpesvirus is transmitted primarily by saliva, though other routes including semen and breastmilk have also been described. There exists a big discrepancy in KSHV seroprevalence across different populations. It is less than 10% in the US, but > 50% in KSHV endemic regions in Sub-Saharan Africa. Whereas the US AIDS-KS epidemic was driven by repeated contact among MSM, in endemic regions KSHV is acquired in childhood. We are responding to RFA-CA-18- 013 to decipher how KSHV is transmitted. The overall goal of this application is to understand the transmission of KSHV in molecular terms, with the ultimate goal being to inform vaccine design and identify other means of disrupting transmission. To achieve this goal, we will investigate the three pillars of transmissibility of infectious agents: (i) variance among KSHV strains using existing specimens from large clinical cohorts, (ii) variance among the innate immune responses to KSHV infection, and (iii) variance in the microbiome which may affect both KSHV itself (as measured by local viral load) and the innate immune response to infection. In each case, we will start by profiling natural human variation and then explore the function of significant viral and host variants in functional assays for entry, replication and transmission.